System And Method For Airspeed Determination

ABSTRACT

A method for determining airspeed of an aircraft that includes determining a rotor model relating a power coefficient of a propeller of the aircraft to an axial inflow velocity through the propeller as a function of a set of rotor operating parameters; determining the set of rotor operating parameters by sampling an electronic control signal associated with an electric motor actuating the propeller; computing the axial inflow velocity through the propeller based on the set of rotor operating parameters using the rotor model; and determining the airspeed based on the axial inflow velocity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation of U.S. application Ser. No.17/467,737, filed 12 Jan. 2021, which is a continuation of U.S.application Ser. No. 16/893,733, filed 5 Jun. 2020, which is acontinuation of U.S. application Ser. No. 16/453,446, filed 26 Jun.2019, which claims the benefit of U.S. Provisional Application No.62/693,232, filed 2 Jul. 2018, each of which is incorporated herein inits entirety by this reference.

TECHNICAL FIELD

This invention relates generally to the flight parameter measurementfield, and more specifically to a new and useful method for airspeedmeasurement in the flight parameter measurement field.

BACKGROUND

Airspeed is one of the most important parameters that an aircraftoperator (e.g., pilot) uses to control an aircraft and understand itsperformance in flight. Failure to accurately indicate the airspeed of anaircraft can lead to improper control inputs being provided by anaircraft operator, component damage, and/or unplanned (e.g.,undesirable) loss of vehicle control and/or the vehicle itself.

Thus, there is a need in the inflight diagnostics field to create a newand useful method for airspeed determination. This invention providessuch a new and useful method.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a flowchart illustration of a method for airspeeddetermination;

FIG. 2 depicts a diagram of a system for airspeed determination;

FIG. 3A depicts a frontal view of an example of a rotor of the systemfor airspeed measurement, and graphical representations of variousparameters utilized in one or more variations of the method for airspeedmeasurement;

FIG. 3B depicts a side view of the example rotor of FIG. 3A, andgraphical representations of various parameters utilized in one or morevariations of the method for airspeed measurement;

FIG. 3C depicts a cross sectional view (at section A-A) of a blade ofthe rotor depicted in FIG. 3A, and graphical representations of variousparameters utilized in one or more variations of the method for airspeedmeasurement;

FIGS. 4A-4B depict an example of an aircraft used in conjunction withthe system and method for airspeed measurement in a hover arrangementand forward arrangement, respectively;

FIG. 5 depicts example tilt angles between the rotor axis and thefreestream velocity vector, consistent with one or more variations ofthe system and method for airspeed measurement;

FIG. 6 depicts an example relative orientation of rotors of an aircraftused in conjunction with a variation of the method for airspeedmeasurement;

FIG. 7 depicts the output of an example implementation of a rotor modeldetermined in accordance with a portion of a variation of the method forairspeed measurement; and

FIG. 8 depicts an example configuration wherein the rotor acts as alifting body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of preferred embodiments of the invention isnot intended to limit the invention to these preferred embodiments, butrather to enable any person skilled in the art to make and use thisinvention.

1. Overview

As shown in FIG. 1 , the method 100 for determining airspeed caninclude: determining a rotor model S100, determining a set of rotoroperating parameters S200, and computing an airspeed based on the set ofrotor operating parameters in combination with the rotor model S300. Themethod 100 can optionally include: computing forces and/or moments onthe aircraft based on the set of rotor operating parameters incombination with the rotor model S350, validating the computed airspeedS400, and/or any other suitable processes and/or blocks related todetermining airspeed.

The method functions to determine the airspeed of an aircraft duringforward motion through the air. The airspeed is preferably defined asthe speed of the freestream relative to the aircraft during flight, butcan additionally or alternatively be otherwise suitably defined. Themethod can also function to determine a model of rotor performance(e.g., a rotor model) to utilize in determining the airspeed (e.g.,among a plurality of models, based on historical data, in real-time,etc.). The method can also function to provide one of a plurality ofindependently-derived airspeed measurements to an aircraft operator(e.g., pilot, autopilot, autonomous agent, remote control system, etc.).However, the method 100 can additionally or alternatively have any othersuitable function.

As shown in FIG. 2 , the system 200 for determining airspeed caninclude: a rotor assembly 210 and a control system 220. The rotorassembly 210 includes a rotor 212 and a drive mechanism 214. In somevariations, the system 200 can include a plurality of rotor assemblies210. The system 200 can additionally or alternatively include any othersuitable components.

The system 200 functions to determine the airspeed of an aircraft (e.g.,an aircraft utilizing a variation of the system 200 for propulsion). Thesystem 200 can also function to propel an aircraft (e.g., by providingthrust via rotary aerodynamic propulsion, simultaneously with providingan airspeed measurement or independently of providing an airspeedmeasurement). The system 200 can also function to provide state feedbackfrom which rotor operating parameters can be determined (e.g., inaccordance with one or more variations of Block S200 of the method 100).

The method 100 is preferably implemented by a system 200 or asubstantially similar system; however, the method 100 can additionallyor alternatively be implemented or executed at any other suitablesystem. The method can be implemented by, in conjunction with, orotherwise in association with various related systems. In particular,the method is preferably implemented at an aircraft 900, wherein one ormore rotor assemblies 210 are attached to the aircraft 900 to act as apropulsion system. The aircraft is preferably electric, and accordinglyeach of the one or more rotor assemblies preferably includes an electricmotor (e.g., wherein the drive mechanism includes an electric motor) anda rotor. However, in alternative variations, the aircraft can be poweredand/or driven by any other suitable drive mechanism or powertrain (e.g.,a non-electric powertrain and/or drive mechanism). The rotor of therotor assembly functions to provide propulsive force to the aircraft(e.g., under actuation by a torque provided by the drive mechanism).

The rotor can have any suitable number of blades; the rotor preferablyhas five blades, but can alternatively have three blades, four blades,six blades, and any other suitable number of blades. The blades can be:rigidly fixed to a hub (e.g., as a fixed-pitch propeller); coupled to ahub and including variable pitch capability (e.g., by way of a suitablevariable pitch linkage, cyclic pitch control, etc.), and/or connected toa hub or rotor head by one or more hinges (e.g., a drag hinge, a flaphinge, etc.) to enable blades to lead, lag, and/or flap relative to thehub or rotor head during rotation of the rotor under aerodynamicloading. However, the blades can be otherwise suitably coupled to oneanother and/or otherwise suitably mechanically linked to form at least aportion of the rotor. In a specific example, the rotor includes fivevariable-pitch blades, wherein the blade pitch angle β is controllableby a system controller.

The term “rotor” as utilized herein can refer to a rotor, a propeller,and/or any other suitable rotary aerodynamic actuator. While a rotor canrefer to a rotary aerodynamic actuator that makes use of an articulatedor semi-rigid hub (e.g., wherein the connection of the blades to the hubcan be articulated, flexible, rigid, and/or otherwise connected), and apropeller can refer to a rotary aerodynamic actuator that makes use of arigid hub (e.g., wherein the connection of the blades to the hub can bearticulated, flexible, rigid, and/or otherwise connected), no suchdistinction is explicit or implied when used herein, and the usage of“rotor” can refer to either configuration, and any other suitableconfiguration of articulated or rigid blades, and/or any other suitableconfiguration of blade connections to a central member or hub. Likewise,the usage of “propeller” can refer to either configuration, and anyother suitable configuration of articulated or rigid blades, and/or anyother suitable configuration of blade connections to a central member orhub. Accordingly, the tiltrotor aircraft can be referred to as atilt-propeller aircraft, a tilt-prop aircraft, and/or otherwise suitablyreferred to or described.

The aircraft is preferably a tiltrotor aircraft with a plurality ofrotor assemblies, operable between a forward arrangement and a hoverarrangement. However, the aircraft can alternatively be a fixed wingaircraft with one or more rotor assemblies, a helicopter with one ormore rotor assemblies, and/or any other suitable aircraft or vehiclepropelled by rotors. The method 100 is preferably implemented using arotor arranged with a non-zero forward tilt (e.g., wherein the thrustvector generated by the rotor during operation has at least a non-zerohorizontal component); however, in variations, the method 100 can beimplemented utilizing any suitable rotor or plurality of rotors in anysuitable arrangement or orientation. The aircraft preferably includes anall-electric powertrain (e.g., battery powered electric motors) to drivethe one or more rotor assemblies, but can additionally or alternativelyinclude a hybrid powertrain (e.g., a gas-electric hybrid including aninternal-combustion generator), an internal-combustion powertrain (e.g.,including a gas-turbine engine, a turboprop engine, etc.), and any othersuitable powertrain.

In a specific example of the system implemented in conjunction with anaircraft, as shown in FIGS. 4A-4B, the aircraft can include six rotorassemblies operable between a forward arrangement, wherein each of therotor assemblies is oriented such that the rotor axis is substantiallyparallel to the longitudinal axis of the aircraft (e.g., as shown inFIG. 4B), and a hover arrangement, wherein each of the rotor assembliesis oriented such that the rotor axis is substantially perpendicular tothe longitudinal axis of the aircraft (e.g., as shown in FIG. 4A). Theaircraft can, in examples, be an aircraft substantially as described inU.S. patent application Ser. No. 16/409,653, filed 10 May 2019, which isincorporated herein in its entirety by this reference. However, inalternative examples, the system can be implemented in conjunction withany suitable aircraft utilizing a rotary aerodynamic actuator andcapable of forward flight under the propulsive force of the rotaryaerodynamic actuator.

However, the method 100 and system 200 can additionally or alternativelybe utilized in conjunction with any other suitable related systems.

As shown in FIGS. 3A, 3B, and 3C, various geometric and physicalquantities and features are related to the method 100 and/or variationsthereof. At a given operating condition (e.g., set of operatingparameters, set of rotor operating parameters, etc.), the rotor rotatesabout the rotor axis at angular velocity ω, and each position along thespan of a blade of the rotor (e.g., at a position r between 0 and R,wherein R is the total length of the blade) experiences an effectiveoncoming flow velocity due to this rotation in the θ direction of ωr, asshown in FIG. 3A. The freestream velocity V_(∞) (e.g., the velocity ofthe air upstream of the aircraft, prior to deflection, deceleration,and/or compression of the air by the aircraft or another aerodynamicbody) reaches the disc of the rotor (e.g., rotor disc) out of the discplane (e.g., normal to the disc plane, oblique to the disc plane, etc.),as shown in FIG. 3B. At each radial position of the blade, the bladedefines a cross section (e.g., an airfoil cross section) such as theexample depicted in FIG. 3C (e.g., section A-A). The blade can beinclined at an angle β relative to the disc plane (e.g., between thedirection defined by the ωr vector and the zero-lift line, wherein thezero-lift line is the axis parallel to which the blade will generate nolift when moving; the angle of attack of the blade; etc.); the bladeangle can additionally or alternatively be defined as a deviation fromthe rotation axis, wherein the rotation axis (e.g., rotor axis) isdefined as the axis about which the rotor rotates during operation.

In some variations (e.g., wherein the system is used in conjunction witha tiltrotor aircraft), the rotor assembly can be configurable between aforward configuration and a hover configuration. Thus, as shown in FIG.5 , the disc plane of one or more rotors can be rotated between a tiltangle of ϕ=0° with respect to the freestream direction and a tilt anglegreater than 0 with respect to the freestream direction (e.g., (=90°)and/or to a hypothetical freestream direction associated with forwardflight in cases wherein the aircraft is not necessarily actively engagedin forward flight (e.g., wherein the aircraft is hovering). Thefreestream direction may be parallel with a ground surface or at anangle with respect to the ground surface, in relation to theaforementioned tilt angles.

2. Benefits

The system and method and variations thereof can afford several benefitsand/or advantages.

First, variations of the system and/or method can enable an aircraft toaccurately measure its airspeed in conditions that may reduce or negatethe effectiveness of conventional airspeed measurement techniques. Forexample, under certain environmental conditions, ice can form on oraround pitot tube inlets and prevent accurate pressure transduction;this can cause an aircraft operator to provide inaccurate control inputs(e.g., control inputs based on an inaccurate airspeed indication) thatcan in turn lead to adverse events (e.g., stall, crash, loss ofstability, loss of efficiency, etc.). The system and method aretypically not susceptible to such environmental conditions, because inany situation in which the aircraft is actively operated (e.g., duringpowered flight) one or more rotor assemblies are necessarily functional,and thus the airspeed can often be inferred in accordance with one ormore variations of the system and/or method.

Second, variations of the system and/or method can provide a redundantairspeed measurement to aircraft equipped with alternative means ofairspeed measurement and indication. In such variations, the systemand/or method can be the primary means of airspeed determination or asecondary means of airspeed determination. For example, the method canbe used to determine the airspeed displayed to an operator (e.g., apilot, an autonomous vehicle control system, a teleoperator, etc.) untiland/or unless a trigger condition is reached (e.g., disagreement betweenthe airspeed determined via the method and via an alternative meansincreases above a threshold disagreement value), at or around whichpoint the alternative mechanism can be employed (e.g., in addition tothe method, in lieu of the method, etc.), or vice versa (e.g., wherein aconventional means is used until environmental conditions rendering theconventional means unsafe or unusable are determined, and one or morevariations of the method is used in addition to the conventional means,in lieu of the conventional means, etc.). In another example, anoperator can utilize a plurality of means of airspeed determination(e.g., simultaneously, in a selectable manner among the plurality, etc.)that includes variations of the system and/or method as well asalternative means. In further examples, sensor fusion techniques can beused (e.g., Kalman filtering), incorporating the inverted rotor model asan airspeed as a “sensor” input, to determine the correct airspeedand/or reject misleading sensor data (e.g., from a malfunctioningconventional airspeed sensor). Redundant airspeed indication systems canimprove aircraft safety, operator performance and/or confidence, andother suitable aspects of aircraft operation.

Third, variations of the system and/or method can remove the need foralternative mechanisms of airspeed measurement and indication. Forexample, implementations of the system and/or method can enable a pitottube network or other alternative direct airspeed measurement system tobe removed from an aircraft, which can reduce aircraft weight, improveaerodynamic performance (e.g., by elimination of drag-producing featuresassociated with the alternative mechanism), reduce complexity ofaircraft system, and provide any other suitable benefits of componentreduction in an engineering system.

Fourth, variations of the system and/or method can leverage the use ofan electric drive train (e.g., an electric motor directly driving therotor) to provide accurate torque output information (e.g., shaft poweroutput of the electric motor as a function of input electrical power)without measuring the torque directly (e.g., using a specific sensorcoupled to the drive mechanism intended to monitor the power output).Accurate knowledge of the torque is utilized in one or more variationsof the method 100, and can be difficult to obtain without dedicatedmeasurement apparatuses when using alternative propulsion systems (e.g.,gas turbine propulsion, reciprocating internal combustion drivenpropulsion, etc.) due to the complex mechanical power transferassociated therewith. In addition, variations of the system and methodcan include using electrical actuators to control all aspects of rotoroperation (e.g., blade pitch angle as a function of time, tilt angle asa function of time, etc.), which can enable the operating parameters tobe determined as a direct function of the control inputs (e.g., voltageinput to a servo motor, signal input to a tilt mechanism, etc.) insteadof via direct or indirect measurement.

However, the system, method, and variations thereof can additionally oralternatively afford any other suitable benefits and/or advantages.

3. Method

As shown in FIG. 1 , the method 100 can include: determining a rotormodel S100, determining a set of rotor operating parameters S200, andcomputing an airspeed based on the set of rotor operating parameters incombination with the rotor model S300. The method 100 can optionallyinclude: validating the computed airspeed S400, and/or any othersuitable processes for determining an airspeed.

The method functions to determine the airspeed of an aircraft duringforward motion through the air. The airspeed is preferably defined asthe speed of the freestream relative to the aircraft during flight, butcan additionally or alternatively be otherwise suitably defined. Themethod can also function to determine a model of rotor performance(e.g., a rotor model) to utilize in determining the airspeed (e.g.,among a plurality of models, based on historical data, in real-time,etc.). The method can also function to provide one of a plurality ofindependently-derived airspeed measurements to an aircraft operator(e.g., pilot, autopilot, remote control system, etc.). However, themethod 100 can additionally or alternatively have any other suitablefunction.

Block S100 includes determining a rotor model. Block S100 functions toobtain a mathematical relationship between the performance of the rotorof the aircraft under various operating conditions and the associatedairspeed of the aircraft, for use in calculating the airspeed of theaircraft during operation based on rotor operating parameters (e.g., inaccordance with one or more variations of Block S200, S300, etc.). Insome variations, the model can directly relate the rotor operatingperformance and characteristics to the airspeed; in additional oralternative variations, the model can relate the rotor operatingperformance to a secondary parameter of the airflow proximal the rotor(e.g., the axial inflow velocity, the flowfield around the rotor actingas a lifting body, etc.) that enables the airspeed to be estimated basedon the secondary parameter of the airflow (e.g., geometrically).However, the model can be otherwise suitably defined such that theoperating characteristics of the rotor are related to the airspeed ofthe vehicle.

The model f is preferably of the form:

f:(M,β,ω)→{tilde over (v)}

where M is the shaft torque (e.g., applied to the rotor, rotor shafttorque, output torque of the electric motor, etc.), β is the blade pitchangle as described above, ω is the rotor angular velocity as describedabove, and {tilde over (v)} is the estimate of the axial inflow velocity(e.g., into the propeller, into the rotor, etc.). In some cases (e.g.,wherein the rotor is lightly loaded), the axial inflow velocity can betaken to be equal to the component of the freestream velocity alignedwith the rotor axis. In additional or alternative examples (e.g.,wherein the rotor is substantially loaded), the axial inflow velocitycan include a component of self-induced inflow (e.g., induced viamomentum exchange between the rotor and the flow during operation), andthe model can include a corrective term to account for the self-inducedinflow in computing the freestream velocity (e.g., and the airspeedpursuant to the freestream velocity). The model can additionally oralternatively be of the form described above and include additionalmodel parameters, including air density ρ and/or the sound speedα=√{square root over (γR_(air)T)}.

The blade pitch used in the model is preferably measured at the μRstation along the span of the blade, wherein μ is the fraction between 0and 1 along the rotor radius R (e.g., span of the blade from the hub orother central member); however, the blade pitch can additionally oralternatively be otherwise measured or determined (e.g., as an averageof the pitch at multiple stations along the span, span-wise to determinea mapping as a function of span position, etc.). In some variations, theblade pitch angle is a function of the station; in alternativevariations, the blade pitch angle is constant (e.g., identical) at eachstation. In a specific example, the blade pitch used in the model isdefined at μ=0.75; however, in related examples, the blade pitch canadditionally or alternatively be taken as the angle at any othersuitable station along the span.

Block S100 can include determining a model for the coefficient of power(e.g., power coefficient, C_(p), etc.) of the rotor, which is a functionof the axial advance ratio λ and the blade pitch angle β. Given anumerical value for C_(p), the shaft torque M can be expressed as:

M=C _(p)(λ,β)ρπω² R ⁵

where ρ is the air density (e.g., predetermined as a function ofmeasurable thermodynamic state variables such as temperature, barometricpressure, etc.), and the axial advance ratio λ is a non-dimensionalparameter defined as the ratio between the estimated axial inflowvelocity V and the linear speed of the blade ωR, i.e.:

$\lambda = {\frac{\overset{\sim}{v}}{\omega R}.}$

Thus, determining a model for the power coefficient functions to providean expression that can be evaluated (e.g., in subsequent Blocks of themethod 100) based on known control inputs (e.g., shaft torque, RPMvalue, blade pitch angle, etc., determined in Block S200) to derive theaxial inflow velocity, which can be used to determine the freestreamvelocity and thereby the airspeed of the aircraft (e.g., in Block S300).

In a first variation, Block S100 includes determining a linearized powercoefficient model (e.g., a linearized model). The linearized model ispreferably utilized (e.g., during vehicle operation) in cases whereinthe rotor slipstream velocity is less than the axial inflow velocity.However, the linearized model can additionally or alternatively be usedin any other suitable cases and/or conditions (e.g., wherein the axialinflow velocity and the rotor slipstream velocity are approximatelyequal, wherein the axial inflow velocity is less than the rotorslipstream velocity, etc.). The linearized model is preferably alinearization of an exact non-linear geometrically-derived model ofC_(p), but can additionally or alternatively be a linearization of anysuitable non-linear model of C_(p).

In the linearized model, it is preferably assumed that the loading(e.g., disc loading, aerodynamic loading, etc.) is applied entirely atthe μR station of the blade. However, in additional or alternativevariations of the linearized model, the loading can be assumed to act onthe blade at any suitable location and/or with any other suitablespatial distribution.

In a specific example of the linearized model, the linearized axialadvance ratio is expressed as:

${{\overset{\hat{}}{\lambda} = {\lambda_{0} - \frac{3C_{p}\sqrt{1 + \lambda_{\mu}^{2}}}{{\pi\sigma\lambda}_{\mu}}}}❘}_{\lambda_{\mu} = {\lambda_{0}/\mu}},$

wherein the power coefficient is defined as

${C_{p} = \frac{M}{\rho\pi\omega^{2}R^{5}}},$

and λ₀ represents the advance ratio corresponding to a freely spinning(e.g., zero loading) condition and is determined via the expression:

λ₀=μ tan(β−α₀),

wherein β is the chosen pitch angle and α₀ is the zero-lift angle of theairfoil defined by the blade geometry at the μR station (e.g., as shownin FIG. 3C). Given the aforementioned parameters, the axial inflowvelocity is given by:

{tilde over (v)}=ωR{tilde over (λ)}.

In the aforementioned expressions, the rotor solidity σ (e.g., the ratioof the total area of the rotor blades to the swept area of the rotor)and zero-lift angle α₀ are geometric parameters of the rotor and bladesthereof; in a specific example, the parameter values corresponding tothe rotor of the aircraft are σ=0.15 and α₀=−2°, but can additionally oralternatively have any suitable values corresponding to any suitablegeometry (e.g., of the rotor or propeller, of the blades, etc.).

As shown in FIG. 7 , the linearized model results in a parameterizationof the inflow ratio λ and power coefficient C_(p), for various values ofthe blade pitch angle β. Given the operating parameters of the rotorassembly (e.g., as determined in one or more variations of Block S200),the inflow velocity can be determined and the corresponding airspeedcalculated (e.g., in one or more variations of Block S300) using thelinearized model.

In another variation of Block S100, the rotor model can be determinedfrom a computational fluid dynamic (CFD) simulation of the rotor. Inthis variation, the rotor operating parameters are simulated (e.g.,rotor geometry, input moments and torques, adjusted blade pitch angles,angles between the disc plane and the freestream direction, etc.) andthe resulting axial inflow velocity is computed as a direct result ofthe simulation. Various CFD techniques can be used in this variation,including: computer aided design (CAD) geometry generation,discretization of the geometry into any suitable mesh (e.g., uniform,non-uniform, structured, unstructured, using any suitable combination ofhexahedral, tetrahedral, prismatic, pyramidal, and/or otherwisepolyhedral elements, etc.), discretization of the governing equationsaccording to any suitable scheme (e.g., finite volume, finite element,finite difference, spectral elements, boundary elements, high resolutionschemes to capture shocks and other discontinuities, flux limitingschemes, etc.), utilization of any suitable boundary conditions (e.g.,physical boundary conditions such as inflow, outflow, porous surface,moving surface, static surface, etc.; temporal boundary conditions suchas initial conditions; etc.), various implemented physical models (e.g.,equations of fluid motion such as the full Navier-Stokes equations,simplified Navier-Stokes, potential flow, etc.; turbulence models suchas Reynolds-averaged Navier-Stokes, species conservation, enthalpyconservation, radiation models, etc.), and any other suitable CFDtechniques and related physical simulation techniques. In this variationof Block S100, the CFD simulation preferably produces a parameterizationof the axial inflow velocity in terms of measurable and/or controllableoperating parameters (e.g., shaft torque, blade pitch angle, rotor RPM,tilt angle, etc.); however, the CFD simulation can additionally oralternatively produce any other suitable output model that can beevaluated to determine the axial inflow velocity and/or associatedairspeed.

In another variation, Block S100 can include determining the rotor modelby manual calibration. Manual calibration can include operating therotor assembly across a parameter space of operating parameters (e.g., arange of blade pitch angles, shaft powers, tilt angles, etc.) andmeasuring the output (e.g., axial inflow velocity, slipstream flowvelocity, aircraft airspeed, freestream velocity, etc.) to generate arotor model that relates the operating parameters and output. Manualcalibration can be performed during aircraft flight (e.g., using anauxiliary airspeed indicator or measurement device to measure theoutput), on the ground (e.g., in a wind tunnel), and/or in any othersuitable manner.

In another variation, Block S100 can include determining a lifting bodymodel of the rotor. In this variation, the rotor model can relate theaxial inflow velocity (e.g., and/or airspeed, directly) to the liftforce generated by the rotor due to the rotor acting as a lifting body(e.g., an airfoil) as compared to the thrust supplied by the rotor(e.g., as shown in FIG. 8 ). This variation is preferably utilized incases wherein the tilt angle of the rotor assembly is such that there isa small angle between the plane of the rotor disc and the freestreamairflow (e.g., wherein the axis of rotation of the rotor is nearlyperpendicular to the freestream airflow, but not entirely perpendicularsuch that there is a non-zero axial flow component through the rotor;wherein airspeed is estimated directly based on the lifting body modelof the rotor and associated forces and moments on the rotor disc; etc.);however, this variation can additionally or alternatively be utilized inany other cases and/or configurations (e.g., wherein the axis ofrotation of the rotor is nearly parallel to the freestream airflowdirection). The lifting body model of the rotor can be determinedanalytically (e.g., modeling the rotor as a symmetric thin airfoil, aflat plate, a bluff body, etc.), empirically (e.g., utilizing a CFDmodel, a wind tunnel, a combination of virtual and physicalexperimentation, etc.), and/or by any other suitable technique.

However, Block S100 can additionally or alternatively includedetermining the rotor model in any suitable manner.

Block S200 includes determining the operating parameters of the rotorassembly. Block S200 functions to obtain the inputs (e.g., operatingparameters) for the rotor model (e.g., as determined in accordance withone or more variations of Block S100), such that the airspeed can becalculated using the model (e.g., in accordance with one or morevariations of Block S300). Block S200 preferably includes determining,for at least one rotor assembly of the aircraft, a set of operatingparameters including: the blade pitch angle of each blade of the rotor,the tilt angle of the rotor assembly, the shaft torque applied to therotor via the drive mechanism of the rotor assembly, and theinstantaneous RPM of the rotor. The set of operating parameters canoptionally include: the air density, other thermodynamic parameters ofthe air from which air density can be computed (e.g., using a table ofthermodynamic variables including two or more of total pressure, staticpressure, density, temperature, specific volume, vapor fraction, etc.),power efficiency of the rotor assembly (e.g., temperature dependentefficiency, electric motor power efficiency, direct drive gearboxefficiency, etc.), time-dependent thermal properties of the rotorassembly (e.g., temperature of various rotor assembly components), andany other suitable operating parameters related to the rotor and/orrotor assembly operation (e.g., rotor operating parameters).

In relation to Block S200, each of the operating parameters ispreferably determined based on the control input provided to the rotorassembly (e.g., by a pilot, automatically provided by a control system,etc.) without active sensing (e.g., without a dedicated sensor ortransducer that independently measures the operating parameter);however, the operating parameters can additionally or alternatively bedetermined via active sensing or otherwise suitably determined. In somevariations, Block S200 can include determining the operating parameterbased on actuator feedback (e.g., back-EMF from an electronicservomotor, a signal from a feedback control loop that controls anactuator, etc.). In additional or alternative variations, the operatingparameter can be determined via direct sensing (e.g., of actual versuscommanded actuator state), indirect sensing (e.g., sensing of relatedactuators that do not directly actuate the propeller state), and/orotherwise suitably determined.

In relation to Block S200, each of the operating parameters preferablycorresponds to an electronic actuator (e.g., an electric motor, anelectric servomotor, a stepper motor, etc.), and thus each operatingparameter can be determined via sampling the electronic control signaland/or power signal associated with the actuator; for example, the rotorRPM and shaft torque can be directly related to the input power to theelectric motor (e.g., that directly drives the rotor), and thusdetermining the rotor RPM and shaft torque can include determining theinput electrical power to the electric motor driving the rotor assembly(e.g., computing the rotor RPM and/or shaft torque via a predeterminedcalibration curve in terms of the input power, voltage, current, etc.;utilizing a predetermined or otherwise known I-V curve of the motor;utilizing a functional relationship between input electrical power andthe output torque and RPM, such as I*V=η(τ*ω), wherein I is theelectrical current, V is the voltage, η is the overall efficiency, τ isthe torque, and a is the angular frequency; otherwise suitablycalculating the RPM and/or shaft torque; etc.). Thus, the method canenable the omission of duplicate signals that encode the operatingparameters (e.g., from transducers that may otherwise be required). Insome variations, forces and/or moments on the aircraft can be measuredby sampling control inputs and/or sensing the state of other actuators(e.g., tilt actuators that transition the propulsion assemblies fromforward to hover configurations, actuators that adjust the position ofcontrol surfaces of the aircraft, etc.). However, in variations whereinsome operating parameters do not correspond to an associated electronicactuator, the operating parameters can be otherwise suitably determined(e.g., via indirect inference).

In relation to Block S200, the set of rotor operating parameters ispreferably determined substantially continuously during flight. Invariations wherein Block S200 includes sampling operating parametersfrom control signals, the operating parameters can be determineddirectly from analog control signals, sampled from digital controlsignals at any suitable frequency (e.g., at a continuous sampling rateabout 100 Hz, between 1-10 Hz, any other suitable frequency, etc.),sampled from digital monitor outputs (e.g., an I/O port of a computingsystem), and/or otherwise suitably sampled, or determined. However, inadditional or alternative variations, Block S200 can be performed inreal-time, substantially real time, according to a schedule (e.g.,associated with various portions of a flight, associated with varioustimes of day, etc.), in response to a trigger event (e.g., failure of anauxiliary airspeed measurement mechanism), and/or otherwise suitablyperformed with any other suitable temporal characteristics.

Block S300 includes computing an airspeed based on the set of rotoroperating parameters (e.g., as determined in Block S200 and/or avariation thereof) in combination with the rotor model (e.g., asdetermined in Block S100 and/or a variation thereof). Block S300functions to transform the set of operating parameters into an airspeedof the aircraft, according to the rotor model. Block S300 can alsofunction to validate the airspeed computation.

In a variation, Block S300 includes computing the airspeed of theaircraft based on the set of rotor operating parameters including theblade pitch angle, the shaft torque output by the electric motor, andthe rotation rate (e.g., RPM) of the rotor, by evaluating the linearizedmodel using the set of rotor operating parameters as inputs. In thisvariation, the axial inflow velocity determined as an output of therotor model is assumed to equal the airspeed. However, in alternativevariations, Block S300 can include correcting the airspeed computation(e.g., by applying a correction factor to account for the slipstreamvelocity and/or any other suitable non-idealities that may affect theassumptions of the linearized model).

In a related variation, Block S300 includes computing the airspeed ofthe aircraft based on the set of rotor operating parameters includingthe tilt angle. In this variation, as shown in FIG. 5 , the rotorassembly can be at a non-zero tilt angle ϕ, and the axial inflowvelocity computed in accordance with the rotor model (e.g., thelinearized model, the nonlinear model, a CFD-derived model, etc.) can bedivided by the cosine of the non-zero tilt angle to obtain the estimatedfreestream velocity (V_(∞)), and thus the airspeed (e.g., assumed equalto the estimated freestream velocity). In implementations of thisvariation, the non-zero tilt angle is preferably less than or equal toabout 60°. In cases wherein a geometrically derived rotor model (e.g.,the linearized model, a nonlinear model based on the rotor geometry,etc.) is used and the angle exceeds about 60° away from the freestreamdirection, Block S300 can include applying a correction factor (e.g., inaddition to the cosine factor) to the computed airspeed to account fordepartures from the geometrically-derived rotor model. In cases whereinthe angle approaches or equals 90°, any other suitable correction can beapplied in lieu of and/or supplemented for dividing by the cosine of thenon-zero tilt angle (i.e., to avoid a divide-by-zero operation).However, Block S300 can include any other suitable manner ofincorporating the tilt angle into the computation of the airspeed.

In another variation, Block S300 can include computing the airspeedbased on the operating parameters corresponding to a plurality of rotorassemblies. For example, Block S300 can include averaging the axialinflow velocities computed in conjunction with two or more rotors of theaircraft, and determining the airspeed based on the averaged axialinflow velocity (e.g., wherein the airspeed is assumed to be equal tothe average axial inflow velocity, equal to the average axial inflowvelocity multiplied by a cosine factor corresponding to each rotorassembly's tilt angle, etc.). In this example and related examples, therotors can all be tilted at the same angle (e.g., all tilted into theforward arrangement as shown in FIG. 4B), at different angles (e.g., asshown in FIG. 6 ), or otherwise suitably arranged.

In a related variation, Block S300 can include determining a weightfactor associated with each of a plurality of airspeeds computed inconjunction with a plurality of rotors. The weight factor can be basedon an error associated with the tilt angle of each rotor, wherein thetilt angles may be different. For example, as shown in FIG. 6 , a firstrotor (e.g., rotor 1) can be configured in a forward configuration whilea second rotor (e.g., rotor 2) is configured at a non-zero tilt angle,and Block S300 can include assigning a larger weight factor (e.g.,corresponding to a smaller error in the airspeed calculation) to thefirst rotor and a smaller weight factor (e.g., corresponding to a largererror in the airspeed calculation) to the second rotor. Accordingly,Block S300 can include computing a weighted airspeed, wherein theweighted airspeed is computed as a weighted sum of the airspeedcomputations associated with each rotor, weighted by the weight factor.Block S300 can additionally or alternatively include selecting fromamong a plurality of airspeed measurements, based on the weight factorof each (e.g., selecting the computation associated with the largestweight factor, selecting the airspeed measurements associated with thetwo highest weight factors and averaging them, etc.). The weight factorcan additionally or alternatively be based on the position of each rotorwith respect to other portions of the aircraft (e.g., wherein a largerweight factor is assigned to rotors arranged towards the nose of theaircraft due to fewer aerodynamic disturbances being present due torotor slipstreams), and/or have any other suitable basis.

Block S300 is preferably performed in substantially real time duringflight, but can additionally or alternatively be performed periodically(e.g., at the same frequency as the determination of the rotor operatingparameters, with any suitable frequency, etc.), asynchronously, inresponse to a trigger (e.g., failure of an auxiliary airspeed indicatoror measurement system, anomalous control feedback triggers a sanitycheck, detection of icing conditions, etc.), and/or with any othersuitable temporal characteristics.

The method 100 can optionally include Block S350, which includescomputing forces and/or moments on the aircraft based on the set ofrotor operating parameters in combination with the rotor model. In somevariations, forces and/or moments can be computed based on the thrustacting upon the aircraft as determined using the rotor model (e.g.,model of the power coefficient of the one or more rotors as a functionof rotor operating parameters substantially as described above). Inadditional or alternative variations, forces and/or moments can becomputed based on the lift acting upon the aircraft as determined usingthe rotor model (e.g., wherein the rotor is modeled as a lifting body,and the lift force on the aircraft from the one or more rotors isdetermined). However, the forces and/or moments on the aircraft can beotherwise suitably computed based on the determined state(s) of therotors of the aircraft.

The method 100 can optionally include Block S400, which includesvalidating the computed airspeed. The computed airspeed is preferably aresult of other portions of the method 100 (e.g., Block S300), but canbe otherwise suitably obtained. Validating the computed airspeed caninclude comparing the airspeed determined via the inverted rotor modelto the airspeed determined or measured via another technique and/or inassociation with another of a plurality of rotors. Validating thecomputed airspeed can include incorporating the computed airspeed into asensor fusion process and assessing the reliability of the computedairspeed. However, Block S400 can additionally or alternatively includeotherwise suitably validating the computed airspeed.

In a first variation, Block S400 can include measuring the airspeed ofthe aircraft using an auxiliary airspeed measurement device (e.g., pitotprobe, air data boom, etc.), and comparing the airspeed measured via theauxiliary device with the computed airspeed (e.g., via Block S300) tovalidate one or the other of the two measurements. In some cases (e.g.,operating conditions, flight conditions, scenarios, etc.), theconfidence level in the computed airspeed based on the rotor model maybe higher than the confidence in the alternatively-measured airspeed,and thus the airspeed computed based on the rotor model can be used tovalidate that the auxiliary airspeed measurement device is functioningproperly. In other cases, the confidence level in thealternatively-measured airspeed may be higher than the confidence in theairspeed computed based on the rotor model, and thus thealternatively-computed airspeed can be used to validate that therotor-model-based airspeed computation is functioning properly. Inexamples of this variation, the confidence level can be expressedquantitatively by a confidence metric. In such examples, Block S400 caninclude determining a confidence metric associated with each determinedairspeed, and validating the determined airspeed associated with theconfidence metric of the lower value using the determined airspeedassociated with the confidence metric of the higher value. However,cross-validation between multiple methods (e.g., independent methods)for airspeed determination can be otherwise suitably performed.

In another variation, Block S400 can include exchanging telemetry with aground station to validate the airspeed measurement. For example, aground station can monitor windspeed and wind heading in the vicinity ofthe aircraft and monitor the ground speed of the aircraft; infer (e.g.,estimate) an airspeed of the aircraft based on the monitored datarelated to the aircraft; receive airspeed measurements from the aircraftas telemetry (e.g., computed in accordance with one or more variationsof Blocks S100, S200, and S300), and validate the received measurementsand/or the inferred airspeed based on a comparison between the twovalues.

In another variation, Block S400 can include combining the airspeeddetermination via the rotor model with additional sensors monitoring thestate of the aircraft, in order to perform cross-validation of thevarious sensor signals. For example, in a sensor fusion scheme thatcombines airspeed information with other aircraft state information,Block S400 can include validating the computed airspeed by combining theoutput (e.g., determined airspeed based on the rotor model incombination with the rotor operating parameters) with other sensoroutput in a sensor fusion process to estimate the state of the aircraft.In such an example, the other sensor inputs besides the airspeed sensor(e.g., a virtual sensor defined by the inverse rotor model computation)can validate the airspeed sensor when the various inputs agree as to thestate of the aircraft; conversely, the agreement (or disagreement) ofthe airspeed sensor with other sensor inputs can validate (orinvalidate) the other sensor data (e.g., to determine whether anotherairspeed sensor, such as an auxiliary airspeed measurement device, isworking properly or not). However, Block S400 can additionally oralternatively include otherwise suitably combining the airspeeddetermination with any other suitable flight data to determine the stateof the aircraft (e.g., including the airspeed of the aircraft).

However, Block S400 can additionally or alternatively include validatingthe airspeed measurement in any other suitable manner.

In a specific implementation, the method 100 includes determining arotor model relating a power coefficient of a propeller of the aircraftto an axial inflow velocity through the propeller as a function of a setof rotor operating parameters (e.g., a blade pitch angle of each bladeof the propeller, a shaft torque applied to the propeller by theelectric motor, an RPM value of the propeller, a tilt angle of thepropeller, etc.). In this example, the method 100 includes determiningthe set of rotor operating parameters by sampling an electronic controlsignal associated with an electric motor actuating the propeller (e.g.,an analog power signal controlled by an onboard computer or othercontroller, a digital control signal input, etc.). Once the set of rotoroperating parameters are determined (e.g., in real time, in near-realtime, etc.), the method includes computing the axial inflow velocitythrough the propeller based on the set of rotor operating parameters(e.g., using the rotor model, by providing the rotor operatingparameters as inputs). In this and related examples, the method 100 canadditionally or alternatively include determining a set of environmentalparameters (e.g., an air density and sound speed proximal the aircraft),and computing the axial inflow velocity through the propeller based onthe set of rotor operating parameters and the set of environmentalparameters; however, in cases wherein the environmental parameters areknown or fall within narrow ranges that do not substantially affect theoutput of the model, environmental parameter values can be assumed(e.g., as a function of known variables, as average values, etc.). Inthis example, the method includes computing the airspeed of the aircraftbased on the axial inflow velocity (e.g., resulting in a determinedairspeed). In cases wherein the aircraft is a tilt-rotor aircraft andthe propeller is tilted relative to the freestream airflow direction,computing the airspeed based on the axial inflow velocity can furtherinclude dividing the axial inflow velocity by the cosine of the tiltangle to geometrically account for the tilt factor. In this example andrelated examples, the method includes measuring the airspeed (e.g.,independently of the inverse rotor model methodology) using an auxiliaryairspeed measurement device (e.g., a ground-based airspeed measurementdevice, an onboard air data boom, another rotor assembly arrangedelsewhere on the aircraft, etc.) to generate a second determinedairspeed, and comparing the first determined airspeed to the seconddetermined airspeed to generate a validation metric. The validationmetric can be used to validate either the first or second determinedairspeed (e.g., based on a confidence level or confidence metricassociated with each determined airspeed), such that in this example andin related examples the method includes validating at least one of thefirst determined airspeed and the second determined airspeed based onthe validation metric.

In cases wherein the auxiliary airspeed measurement device includes aground-based airspeed measurement device, the method can includeexchanging telemetry between the aircraft and a ground station (e.g., togenerate the second determined airspeed). In such instances, the methodcan include monitoring windspeed and the wind heading proximal theaircraft at the ground station (e.g., using an instrument at the groundstation), as well as monitoring the effective ground speed and headingof the aircraft relative to the ground station (e.g., using aninstrument at the ground station such as a radar), in order to determinethe airspeed of the aircraft (e.g., the second determined airspeed tovalidate the first determined airspeed or be validated by the firstdetermined airspeed).

Although omitted for conciseness, the preferred embodiments includeevery combination and permutation of the various system components andmethod blocks, which can be combined in any suitable permutation orcombination and/or omitted in whole or in part from variations of thepreferred embodiments. No ordering or sequence is necessarily impliedother than where explicitly stated with respect to method blocks and/orprocesses; Blocks of the method 100 and variations thereof can berepeated, performed iteratively, and/or executed in any suitable order,in addition to being omitted in whole or in part in variations of themethod 100.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

1. A method for an aircraft comprising: determining a set of propelleroperating parameters based on an electronic control signal controllingactuation of a propeller; determining an inflow velocity through thepropeller based on the set of propeller operating parameters and apropeller model relating a power coefficient of the propeller of theaircraft to the inflow velocity through the propeller; and determining avehicle state parameter of the aircraft based on the inflow velocitythrough the propeller.
 2. An aircraft based system for determiningairspeed, said system comprising: an aircraft; one or more rotorassemblies, the rotor assemblies comprising: a propeller; and anelectric motor; a control system housed within the aircraft, the controlsystem configured to determine a first measurement associated with avehicle state associated with an aerodynamic actuator, determine a setof rotor operating parameters based on an electronic control signalassociated with an aerodynamic actuator, calculate a first value of anairflow parameter using a model relating a power coefficient of theaerodynamic actuator to an airflow parameter, and to determine thevehicle state parameter of the of the aircraft based on the firstmeasurement and the first value.
 3. The system of claim 2 wherein theaerodynamic actuator comprises a rotary aerodynamic actuator.
 4. Thesystem of claim 2 wherein the airflow parameter comprises an axialinflow velocity.
 5. The system of claim 2 wherein the vehicle stateparameter comprises an airspeed of the aircraft.
 6. The system of claim2 wherein the vehicle state parameter comprises at least one of a forceon the aircraft or moment on the aircraft.